This invention relates to filters, and in particular to a high shear rotating disc filter.
Current filtering technologies cover a wide gamut of designs for a wide variety of liquid/solid separating applications. Solids refer to small particles as in powders. These technologies are necessitated by the many filtering characteristics of the liquid/solid mixtures or slurries which are being separated. a slurry being defined as a liquid solid mixture. Such characteristics will include particle size distribution, particle shapes, particle hardness/softness, solids concentration, liquid viscosity, flow rate slurry temperature, degree of separation required, etc. However, each filtering technology has its operation and economic limitations. Commonly used filters include: filter press with and without membrane bladders, candle filters, leaf filters (vertical and horizontal), nutsche type filters, rotary vacuum filters, vacuum disc filters, etc.
With each of the commonly used filters, the general mode of operation might include some of the following steps:                (i) pre-coating the filter leaves with filter-aid such as diatomaceous earth (DE);        (ii) introducing the slurry to be filtered (may also contain filter-aid in the form of a body feed);        (iii) separating the liquid and solid phases via vacuum or pressure, continuing until resistance to filtrate (clean filtered liquid) flow increases to a specified pressure level (a sign that the open areas between the solid particles have been greatly reduced and thereby choking the flow of the filtrate; blinding, i.e., filtrate surface being made impervious, is effectively occurring and thereby advancing the cycle to its next step such as washing, squeezing, discharging, back-washing, etc.);        (iv) discharging the remaining slurry within the filter chamber (typically recycled back to the feed tank as unfiltered slurry);        (v) passing a high volume of air through the filtered cake to dry the filtered solids (some filters will employ added mechanical pressure to squeeze the solids to a drier consistency);        (vi) discharging the accumulated solids; and        (vii) reclaiming semi-dry and wet solids at the bottom of the filtration vessel and disposing the solids along with any filter aid used.        
With some filtering technologies, such as a rotary vacuum drum filter and a vacuum disc filter, the liquid-solid separation is accomplished by dipping a part of the drum or disc into the slurry while applying a vacuum to draw the liquid through the thin layer of cake, past the filter medium and out of the vessel via a series of internal pipes. As the drum/disc rotates, the formed cake on the surface of the medium will move out of the wet slurry and is then exposed to the air for more drying by added suction (vacuum) until it reaches a point at which the cake is discharged from the surface by a variety of means such as discharge blades or pneumatic blow-back. As the drum/disc continues to rotate it will then again dip into the slurry and the cycle starts all over again.
One other commonly used technology for the separation of these slurries is that of centrifuging, i.e., decanters and disc-stack centrifuges. The high rotational speeds of these units will generate forces up to 15,000×G's. The centrifugal forces will effectively separate the liquid/solid slurries due to the specific gravity differences between the solids and liquids.
Each technology has its limitations based on a variety of factors that include: specific gravity differences, concentrations, viscosities, solids characteristics, etc.
The present invention is primarily applied to slurries that contain low solids concentrations and small particle size distributions, though not limited solely to these applications. The characteristics of the solids will generally be somewhat soft and have a particle size of less than 20 microns to possible sub-micron. Historically, these slurries have been filtered with the assistance of filter-aid such as DE (diatomaceous earth) since the solids being filtered have a tendency to blind the filter medium soon after the initial solids layer is deposited on its surface. The intent of the filter-aid is to assist in preventing “blinding” of the filter medium by providing more surface area on which the solids can rest while maintaining a more open flow for the filtrate. However, if the DE is used as a pre-coat or body feed, it will eventually have to be discarded with the accumulated solids which will add to the cost of the operation. In cases where the solids are a valuable product, DE cannot be used since it will contaminate the solids.